1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor element, and especially a semiconductor element which has metallic silicide layer in its gate electrode.
2. Description of the Related Art
As high integration of semiconductor devices proceeds, a demand for faster and finer elements of semiconductors increases. To meet this demand, a gate electrode comprising a metallic silicide layer is adopted for a semiconductor element of MOS structure.
A gate electrode having a metallic silicide layer is formed, for example, as described below.
At first, an isolation of an element is performed. After this, a gate oxide film and a polysilicon film are formed, in this order. Then, a metallic silicide layer is formed on the polysilicon film by a PVD method or a CVD method. A SiN layer is formed on top of the gate electrode with a thermal process using CVD of reduced pressure etc. This thermal process is performed at a comparatively high temperature in a range of 700° C. to 800° C.
After forming in SiN layer, a lithography for forming a gate electrode is performed. In addition, an etching process for forming a gate electrode is performed with this patterned SiN layer used as a mask. Thus, a gate electrode is formed. After this, the photo-resist used for etching is removed.
After removing the photo-resist, oxide films are formed for covering the side walls of the gate electrode with a CVD method of reduced pressure etc.
However, the conventional structure of the gate electrode mentioned above could not sufficiently recover from the damage received during the etching process, because the side wall of the gate electrode was covered with oxide film by a CVD method of reduced pressure. This is a problem to be solved. Moreover, over-etching of the gate oxide film occurs. Consequently, it becomes impossible to compensate for thinning of the gate oxide film. As a result, there was another problem of causing leakage of the gate.
Therefore, in order to prevent leakage of the gate, a method of performing heat oxidation treatment is generally used. But, in the method mentioned above, a heat treatment for forming SiN layer was necessary. Therefore, the metallic silicide was crystallized, and, the crystal was in a state that oxygen was likely to diffuse. An oxidation process in this state caused increasing oxidation of metal of W. and Si, and expansion of volume occurred. Therefore, difference of stress occurred between polysilicon and metallic silicide in the lower portion of the metallic silicide layer. A difference of stress also occurred between the metallic silicide and SiN layer in the upper portion of metallic silicide layer. As a result, fastening between the metallic silicide layer and SiN deteriorated, and peeling off of the SiN layer occurred.
That is, according to the method mentioned above, a gate electrode 40 comprising polysilicon layer 42, metallic silicide layer 43, and SiN layer 44 was formed. When its surface was observed, the state of the surface shown in FIG. 5(b) was recognized. In FIG. 5(b), metallic silicide crystal 45 grew like notches in the horizontal direction, because the side wall of gate electrode 40 was abnormally oxidized. In addition, SiN layer 44 was likely to peel off due to abnormal oxidation as mentioned above.
As a method of preventing abnormal oxidation mentioned above, a method of implanting nitrogen in the side wall of gate electrode 40 has been suggested (c.f. JP 08-321613).
However, the conventional method has a problem in that voids (vacancies) occur because of nitrogen implanted in the semiconductor substrate.